The present invention relates generally to web and sheet handling apparatus, and more particularly, to such apparatus in which synchronization of web and or sheet movement is required. The invention enables a rotational encoder having sufficient accuracy for single web operations to be used with similar accuracy over a long run of similar operations.
A number of web-handling devices are known in which proper registration of the web with another web, with sheets, or with the device itself is required. The exact function of these devices depends upon the nature of the web material which is to be handled, as well as the particular operation to be carried out.
One type of web material is disclosed in U.S. Pat. Nos. 4,440,846 and 4,399,209, which describe an imaging media for use with an imaging system. In the media, a photosensitive layer is formed comprising microcapsules containing a photosensitive composition in the internal phase. The photosensitive layer is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. An image-forming chromogenic material, such a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing image-wise exposed imaging sheets through the nip between a pair of parallel calender rolls.
This photosensitive media may exist in a form which comprises two sheet-like layers. The microcapsules are carried on a first substrate layer, which layer is most conveniently provided in the form of a continuous web referred to as a donor web. The developer composition is coated onto a second, separate substrate layer, typically formed as separate sheets, with each sheet referred to as a receiver sheet. The donor web is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheets. The sheet and web are developed by pressure, with the finished image being formed in the receiver sheet.
It will be readily recognized that when the receiver sheet is brought into contact with the donor web, the receiver sheet must be accurately registered with the web, otherwise the finished image will be offset on the sheet. Once the donor web has been exposed to actinic radiation, the location of the image on the web has been fixed. Thus, the receiver sheet must be properly positioned with respect to the web in order to correspond to the exposed image.
A variety of registration systems are known for devices which operate on moving webs. In general, for such registration systems to operate properly, accurate data concerning the positioning of the web must be available. For example, in the case of the photosensitive media described above, both the positioning of the exposed portion of the web with respect to the web, and the overall positioning of the web with respect to the apparatus must be known in order for the sheet to be brought properly into position.
One common means for monitoring the positioning of a moving web with respect to a rotating member is to connect a rotary positional encoder to the rotating member. In the case of apparatus operating on the microencapsulated photosensitive media, such a member could be, for example, one of the pressure rolls used to develop the donor web and receiver sheets to produce finished images. Encoders for such use are well known, available with varying degrees of accuracy. As might be expected, the degree of accuracy of the encoder is typically closely related to the cost of the encoder.
In selecting an appropriate encoder for use in registering operations on a moving web, encoder accuracy must be considered from two standpoints. First, the encoder must have sufficient accuracy for a single given operational step, and second, it must have sufficient accuracy over a long series of operations. Many commercially available encoders can provide a necessary single-step accuracy for even precise applications. For example, for the microencapsulated photosensitive media, any encoder accuracy better than 0.004" provides the necessary single-step accuracy in registering the receiver sheet to the donor web. Such encoders are available at reasonable cost. However, over a long run, this accuracy is not sufficient. In the course of incrementing the imaging system for the media five times to produce five images, cumulative error of as much as 0.020" could occur. Such amount of error may produce unsatisfactory results.
What is needed, therefore, is a means for precisely identifying a series of predetermined lengths along a web in successive fashion. Such means should operate so that lengths are determined as a function of the rotation of an element operating on the moving web. Such means should be capable of providing an accuracy sufficient for registering operations on the web, particularly where the web represents a donor material to which a receiver sheet must be registered. Further, despite the capability for sufficient accuracy over a long run, the length determining means should be of a reasonable cost and complexity.